Polymers self-assemble to form 2.5-nm diode

CHICAGO  A diode measuring just 2.5 nanometers was recently demonstrated by University of Chicago professor Luping Yu, who called it the world's smallest. The operation of the polymer-based p-n junction diode, synthesized using organic chemistry by postdoctoral assistant Man-Kit Ng, was verified with a scanning tunneling microscope.

"We did not invent the p- or n-type materials, but we are the first to successfully put them together into a diode," said Yu. "It's much more difficult than it sounds. I owe it all to Man-Kit's excellent work in the lab."

According to Yu, organic chemists can synthesize almost any polymer-based electronic component they can conceive. The problem is structuring the devices in a circuit. While a chemist can coax the components into self-assembling in a beaker, they are just left floating around disconnected, of no use to anyone.

"We feel assured that we can synthesize three-terminal transistors out of these or similar materials, but we have yet to find a way to attach leads to their terminals," said Yu.

Lab savvy

For the diodes, Yu and Ng solved the problem with good old laboratory savvy. Yu chose diblock co-polymer-based materials, each doped to have opposite electrical properties  essentially, p-type and n-type polymers.

In the reactions, electron-rich bithiophene was bonded to electron-poor bithiazole. Then a disulfide was added to the thiophene end, making it possible to connect the two-part molecule to a gold substrate.

The method worked by taking the beaker full of diodes and forming a thin film with them. Then, using normal chemistry steps, the diodes were compressed into a densely packed monolayer of like-oriented diodes end-up, with one end attached to a gold substrate.

To verify that the molecule arrays behaved like traditional diodes, only on a nanoscale, the researchers brought in a scanning tunneling microscope to touch the dangling end of the diodes. They found the diodes turned on at about 0.8 volt, as expected, and that their impedance was between 60 and 300 gigohms.

"I don't want to overstate the importance of our discovery, but we think that many other types of molecular electronic devices can be built in this way, such as arrays of molecular LEDs," said Yu.